It is known that nanoparticles on the scale of tens of nm (comparable to a virus) will move through blood vessels towards tumors in mammals, are unlikely to pass through the blood vessel walls and infiltrate healthy tissue and tend to accumulate easily in areas of abnormal blood vessel formation in the vicinity of tumors. As an example of nanoparticles having this effect, the inventors succeeded in effectively encapsulating anti-tumor agents and nucleic acid substances (genes (plasmid DNA) and gene-suppressing RNA chains (siRNA)) in a polymer micelle created by self-organization of a specific polymer, thereby providing a drug-encapsulating polymer micelle (see for example Patent Documents 1, 2 and 3 below).    Patent Document 1: U.S. Pat. No. 7,125,546A    Patent Document 2: EP 1621569A    Patent Document 3: WO 2006/085664